1. Field of the Invention
The present disclosure generally relates to the field of amplifier circuits, and in particular but not exclusively to a low noise level amplifier circuit.
2. Description of the Related Art
Low noise amplifier circuits are frequently used in the field of telecommunications and in particular in designing telephone interface circuits.
With some applications, it can happen that a relatively large input impedancexe2x80x94about several kilo-ohmsxe2x80x94must be provided for. Such an impedance value is likely to generate non-negligible noise on the amplifier""s input since noise varies in increase ratio to the square root of the input impedance. To minimize the effects of noise, a Low Noise Amplifier structure is then used, that is based on amplifiers mounted as cascode circuits, as illustrated in FIG. 1. A first and a second differential amplifier 110 and 120, receive a signal, respectively INP and INN, on their respective positive input via a bypass capacitor C, respectively 114 and 124. Differential mode gain is set by a voltage divider bridge R1-R2, respectively 130-140 for amplifier OA1 and 150-160 for amplifier OA2, that makes it possible to feed part of the output voltage (resp. OUTP and OUTN) back into the switch input of the amplifiers.
A resistor 100 having a value R is connected between the positive input of OA1 and the positive input of OA2 and makes it possible to set the circuit input impedance.
The noise generated by resistor 100 is filtered through network R-C resulting from the presence of bypass capacitor C (respectively 114 and 124) before it reaches the inputs of the amplifiers. For this reason such an amplifier structure, based on stages mounted as a cascode circuit, proves to be particularly adapted to design amplifiers having a large input impedance.
Nevertheless, the known circuit of FIG. 1 faces a stabilization problem for both amplifiers 110 and 120. Indeed, to avoid them from starting to oscillate at high frequencies, the amplifier""s gain is made to drop down when approaching a critical phase shift of 180 degrees. This gain drop is classically operated by means of a capacitor Cm, known as a Miller capacitor, respectively 111 and 121 in FIG. 1, and more detailed in FIG. 2. In FIG. 2, a conventional differential amplifier structure comprising a first stage formed by a differential pair 112-113, a power source 114 and a current mirroring circuit 115-116 is shown. A second stage comprises a transistor 117, for example a MOS-type transistor, and a power source 118. Generally, the Miller capacitor is connected between the input and the output of the last stage, i.e., in the circuit of FIG. 2, between the grid and the drain of transistor 117. Gain can thus efficiently drop when approaching the critical zone where output and input signals are phase-shifted by 180 degrees. It is observed that connecting a capacitor Cmc 119 between grid and voltage Vdd also allows to obtain gain drop, but with quite less effectiveness than with a Miller capacitor. Because of the presence of gain K of the last stage, a capacitor Cmc equal to Cmxc3x97K would be necessary to obtain two equivalent effects and, for this reason, a Miller capacitor is rather preferred to obtain amplifier stabilization.
Generally, this capacitor Cm is dimensioned according to the gain of the stage to stabilize. The lower the gain, the larger the value of this capacitor must be. The circuit of FIG. 1 however has a gain that is different according to whether it operates in differential mode or in common mode. Indeed, in differential mode gain is set by the ratio of resistors, while in common mode, gain is equal to 1.
Stabilizing the circuit for common mode thus means choosing a capacitor Cm having a large value, whereas a much lower value could be chosen in differential mode, in particular in order to preserve the amplifier""s gain-band product. Thus a dilemma arises: either stabilizing the circuit of FIG. 1 for both common and differential modes, and in this case the largest capacitor value is chosen, which results in performance degradation in differential mode, or stabilizing only the differential mode in order to maintain performance in this mode, and then facing stability problems for the common mode.
FIG. 3 shows a known way of solving this problem. An amplifier circuit is based on two amplifiers 310 (OA1) and 320 (OA2) that are mounted as cascode amplifiers by means of a network R1-R2 made up of resistors 330-340 and 350-360, respectively. Two inputs, respectively INP and INN, are connected to the positive input of OA1 via a capacitor 314 and to the positive input of OA2 via a capacitor 315. The input impedance of the circuit is set by a resistor 300. Contrary to the circuit of FIG. 1, voltage VCM of the divider bridge""s midpoint is now set, at the junction between resistors 340 and 350, by means of an amplifier 370 (OA3) that is mounted as a cascode circuit. This circuit has a positive input that is connected to the midpoint of a resistor bridge (Rs 391 and 392), having a voltage stabilized at low frequency by a capacitor 393. If a sufficiently large value C of capacitor 393 is chosen, the output of amplifier 370 is more or less stabilized and thus voltage VCM is set to virtual ground.
Thus, for both stages 310 and 320, a common mode gain can be obtained that is identical to the differential mode gain, which makes it possible to stabilize amplifiers OA1 and OA2 in both modes and with an optimal value when considering the gain-band product. Indeed a single value Cm, when judiciously selected, makes it possible to obtain stabilization in differential mode and in common mode without any loss of performance.
This is the conventional way to stabilize both amplifiers OA1 and OA2. However, it can be observed that the stabilization problem is just transferred to the third amplifier OA3, that must also be associated with a Miller capacitor 380 that will have to be particularly effective, and in particular when approaching the critical operation zone for stages 310 and 320. Indeed, it will be in this zone that amplifier OA3 will be particularly used and thus likely to output large currents to maintain voltage VCM to virtual ground. Besides, the existence of an offset will amplify currents, especially as resistor R1 will have a low value. Designing amplifier OA3 is thus particularly delicate to do.
It is therefore desired to design a new low noise amplifier structure allowing to obtain stabilization more easily, in common mode as well as in differential mode.
One embodiment of the present invention provides a low noise amplifier structure that is easy to stabilize in common mode as well as in differential mode, and without loss of performance.
Another embodiment of the invention provides a low noise amplifier circuit that consumes less current and occupies less room.
An embodiment provides an amplifier structure including:
a first amplifier comprising at least one input stage and one output stage;
a first Miller capacitor having a first electrode and a second electrode, said first and second electrodes being connected to the input and the output of said first amplifier""s output stage, respectively;
a second amplifier comprising at least one input stage and one output stage;
a second Miller capacitor having a first electrode and a second electrode, said first and second electrodes of the second Miller capacitor being connected to the input and the output of said second amplifier""s output stage;
wherein the amplifier structure comprises:
at least a first trimming capacitor having a first electrode and a second electrode, said first electrode being connected to said first electrode of said first Miller capacitor;
at least a second trimming capacitor having a first electrode and a second electrode, said first electrode being connected to said first electrode of said second Miller capacitor;
a cascode stage having an input and an output, said cascode stage input being connected to the midpoint of a resistive bridge connected between the output of said first amplifier and the output of said second amplifier; said cascode stage output being connected to the second electrode of said trimming capacitors.
An effective compensation of each amplifier is thus obtained through combination of Miller capacitors and trimming capacitors, which leads to a discriminated effect in common mode, and in differential mode. The amplifier loops can thus be stabilized in both modes, while preserving a high gain-band product in differential mode.
Indeed, in common mode, the output of the cascode stage follows outputs OUTP and OUTN, which comes down to connecting the trimming capacitors in parallel with the Miller capacitors associated thereto. A more effective stabilization is thus obtained that is equivalent to a single Miller capacitor of value Cm+Cmc.
On the contrary, in differential mode, the output of the cascode circuit remains virtually set to ground, which destroys the Miller effect for both trimming capacitors. Thus, that comes down to having a Miller capacitor having a value Cm+Cmc/k, where K is the gain of the last stage.
Trimming of both amplifiers is thus obtained, which is different according to whether one operates in common mode or in differential mode. In common mode, both amplifiers can be stabilized by judiciously choosing values Cm+Cmc while in differential mode, the effects of an equivalent capacitor Cm+Cmc/k will be at least substantially satisfactory since in that mode it is important to preserve performances in the whole desired signal bandwidth.
Moreover, it is observed that the cascode stage does not have to output any D.C. current, which makes it even easier to design. Thus, the disadvantages of the particularly delicate to conceive known circuit are avoided. In a particular embodiment, the trimming circuit can be realized by a MOS-type transistor mounted in series with a power source to constitute a cascode stage having an output connected to the second electrode of said first and second trimming capacitors.
A first, a second, a third and a fourth resistors are connected in series between outputs (OUTP) and (OUTN) of said first and second amplifiers. The first amplifier has a positive input receiving input signal INP and a negative input that is connected to the midpoint between the first and second resistors. The second amplifier has a positive input receiving input signal INN and has a negative input that is connected to the midpoint between the third and fourth resistors.
In another embodiment, two variable potentiometers are connected in series between the outputs of the first and second amplifiers to obtain a variable gain amplifier circuit.
In an embodiment, each amplifier is associated with a single trimming capacitor by a compensating circuit comprising an associated Miller capacitor.
Alternatively, a set of trimming capacitors associated with switches can be provided to allow perfect trimming according to the gain of the amplifier.
Thus, in any circumstance amplifiers are stabilized, whatever their gain is, and the place occupied by the trimming capacitors can be reduced.
Alternatively, current consumption can be reduced by replacing the third amplifier with two trimming circuits each including a capacitor bridge.
More specifically, one embodiment of the invention also allows to realize a low noise amplifier circuit including:
a first amplifier comprising an output stage;
a first Miller capacitor having a first electrode and a second electrode, said first and second electrodes of said first Miller capacitor being connected to the input and the output of said output stage of the first amplifier;
a second amplifier comprising an output stage;
characterized in that it comprises:
a first trimming circuit including:
a first capacitor comprising a first electrode and a second electrode, the first electrode being connected to output (OUTP) of said first amplifier;
a second capacitor comprising a first electrode and a second electrode, said first electrode of said second capacitor being connected to said second electrode of said first capacitor and to said first electrode of said second Miller capacitor; said second electrode of said second capacitor being connected to output (OUTN) of said second amplifier;
a second trimming circuit including:
a third capacitor comprising a first electrode and a second electrode, the first electrode being connected to output (OUTP) of said first amplifier;
a fourth capacitor comprising a first electrode and a second electrode, said first electrode of said fourth capacitor being connected to said second electrode of said third capacitor and to said first electrode of said first Miller capacitor; said second electrode of said fourth capacitor being connected to output (OUTN) of said second amplifier.